1. Field of the Invention
This invention relates to systems for detecting the occurrence of the leading and trailing edges of pulses and in particular of those pulse-like signals applied by a pacemaker to a patient's heart.
2. Description of the Prior Art
Heart pacemakers such as that described in U.S. Pat. No. 3,057,356, issued in the name of Wilson Greatbatch and assigned to the assignee of this invention, are known for providing electrical stimulating pulses to a patient's heart whereby it is contracted at a desired rate in the order of 72 beats per minute. A heart pacemaker is capable of being implanted in the human body and operative in such an environment for relatively long periods of time, to provide cardiac stimulation at relatively low power levels by utilizing a small, completely implanted, transistorized, battery-operated pacemaker connected via flexible electrode wires directly to the myocardium or heart muscle. The electrical stimulating pulses by this pacemaker are provided at a relatively fixed rate.
Typically, such heart pacemakers are encapsulated in a substance substantially inert to body fluids, and are implanted within the patient's body by a surgical procedure wherein an incision is made in the chest beneath the patient's skin and above the pectoral muscles or in the abdominal region, and a pacemaker is implanted therein. Due to the inconvenience, expense and relative risk to the patient's health, it is highly desired to extend the life of the power source or battery, whereby the number of such surgical procedures is limited. The resultant problem for the attendant doctor is to determine when the batteries should be replaced, keeping in mind the relative risk or probability of premature pacer failure due to battery depletion.
After surgical implantation of an artificial heart pacemaker by known surgical techniques, the patient is required to have periodic checkups so that the heart pacemaker function may be monitored for possible battery or other failure.
A major problem with these devices is that battery failure is not precisely predictable statistically and while statistics do exist they are unfortunately gathered after pacemaker failure has occurred. Further, present heart pacemakers available have a functional life expectancy of about two to five years, but individual ones may not exceed this, and in fact may rather unpredictably fail before this statistical determined period. Usually approximately 90 percent of heart pacemaker failures are battery failures, and the remaining 10 percent are a result of other types of failures, the next most common failure being in the leads themselves. Electronic component failure in artificial cardiac pacers is generally a very small factor. However, all of these factors must be considered when diagnosing a possible malfunction.
The number and kinds of variables that exist make an accurate a priori prediction of the lifetime of a given heart pacemaker difficult. The problem therefore is that of measuring the heart pacemaker pulse, the interval between impulses and some characteristics or set of characteristics which will allow determination in advance of a critical situation, i.e., when the heart pacemaker is about to fail.
One such characteristic is that as the battery starts to fail, the voltage output of the pulses starts to drop and generally as the voltage drops the width of the pacemaker pulse changes. Further, in most heart pacemakers the rate of firing changes.
Most pacemakers generally are powered by 4 or 5 miniature batteries. Present monitoring techniques are geared to detect when the first of those 5 batteries has failed, which means that the safety factor is decreased. In any event, a failure of not only one cell but generally two can be tolerated before the patient is in any danger. It should be cautioned however that when a battery does fail, it fails very rapidly. The battery voltage remains almost constant throughout the lifetime of the battery. Therefore, changes may be detected in the pacemaker output pulses by comparing measurements from one checkup to another.
In one approach to the problem of accurately determining battery depletion, pacemakers such as described in U.S Pat. No. 3,842,844 are provided with a battery or cell depletion indicator that increases the pulse width of the output signal as their batteries deplete, i.e., their voltage amplitude decreases. Further, as the power source or battery depletes, the pulse repetition rate of such artificial cardiac pacemakers also decreases. For example, at the time of implantation, a heart pacemaker may produce stimulating pulses at 70 beats per minute (BPM), plus or minus two beats, with a pulse width in the order of 0.5 msec. After a period of service illustratively in the order of 2-4 years, the BPM changes in the order of 5-10%, i.e., a decrease of 5-7 beats from the original BPM, and the pulse width may increase to a value in the order of 1 msec. Dependent upon the known histories of such batteries, such a change in the BPM as well as a change in pulse width indicates that one of a plurality (e.g. 4 or 5) cells has failed, and that it is time to replace the batteries within the implanted pacemaker to assure continued heart stimulation of a sufficient level.
Pulse width increase is desired to order that as the amplitude of the voltage provided from the pacemaker battery decreases, the total energy in the stimulating pulse remains substantially constant. It is understood that the voltage level of the pacemaker battery may decrease below a level at which the heart may not respond regardless of the pulse width. Further, as the pulse width increases to compensate for decreases in the voltage level, the current drain upon the battery increases, thereby increasing the rate of battery depletion.
In order to detect the patient's electrical heart activity, electrodes are attached for example to the patient's body including his right arm, left arm, left leg, chest and right leg. The electrical activity, as shown in FIG. 2, includes the patient's ECG signal upon which is impressed the pacer pulse appearing before the QRS wave form, which is generated naturally by the heart's activity. The pacer pulse is usually of large amplitude and very small width. Though noting that it is desired to measure the width of the pacer pulse, it is very difficult to determine the width with accuracy. Conventional monitors, for example, do not have a sufficient band width to pass the pacer pulse with any reasonable degree of fidelity. Further, it is necessary to distinguish the pacer pulse from the patient's QRS wave, as well as 60 cycle noise and muscle artifacts. One of the distinguishing characteristics of the pacer pulse is that it has a very fast rise time, being typically in the microsecond range. By contrast, the electrical impulses normally originated in the heart or other noise sources such as 60 Hz line noise, have rise times in the order of 10-20 milliseconds. Other common noise sources may be generated by electrical appliances being operated from the same power line. These generally have fast rise times but very short duration. Since these individual pulses generally are only of fractions of microseconds long, they are distinguished from heart pacemaker pulses principally by the pulse widths since heart pacemaker pulses are commonly in the 1 millisecond range. Thus, the heart pacemaker pulses may be identified by their relatively fast time and their relatively long pulse width in the order of 0.5-4 milliseconds.
In U.S. Pat. No. 3,885,552 of Kennedy, there is disclosed a cardiac monitoring system for monitoring the heart activity and in particular for measuring among other parameters the width of the heart pacemaker pulses. In particular, there is disclosed a pacer pulse selection logic circuit including a differentiator having a time constant of approximately 100 microseconds for providing an output if the applied input has a rise time shorter than 100 microseconds. Further, the noted logic circuit also includes an integrator circuit providing a signal going low indicating that the pulse width of the applied signal is greater than a selected minimum pulse width of 250 microseconds. The Kennedy circuit functions to provide an output identifying the presence of a pacer pulse upon the occurrence of both a signal from the aforementioned differentiator and integrating circuits. For a similar disclosure of a system for detecting the presence of a pacer pulse, attention is also drawn to U.S. Pat. No. 3,871,363 of Day which similarly discloses the use of a differentiator circuit and an integrator circuit for respectively measuring rise times smaller than a selected minimum and pulse widths in excess of a predetermined width to identify thereby heart pacemaker pulses.
Reference is made to FIG. 5, which generally shows an implanted pacemaker of the type generally described above having an output capacitor C3 that is coupled by suitable electrodes to the patient's heart, which for the sake of simplicity may be considered as presenting an essentially resistive impedance to the output of the heart pacemaker. In typical operation, the pacemaker generates a series of timing pulses applied to the base of transistor Q thereby rendering this transistor Q conductive and "dumping" the charge established upon capacitor C3 across the heart's resistance R2. It is recognized, that such a circuit is essentially a differentiation circuit, whereby the electrical signal discharged across the resistor R2 has an essentially sloping or curved wave-form as indicated in FIG. 4A. As shown in FIG. 4A, the pulse appearing between times t1 and t2 has a very fast rise time beginning at time t1, generally sloping down to time t2 with the fast fall time at that instant; this pulse has relatively sharp, well defined leading and trailing edges making its detection relatively easy. However, in practice, the detected wave shape of a pacemaker pulse is more as shown in the time interval between t3 and t5, of FIG. 4A. Generally, the differentiation process effected by the output capacitor C3 and the heart's resistance R3 accounts for the relatively poor wave shape quality, i.e., attenuated quality of this pulse. Thus, it may be observed that the leading edge is relatively well defined and thus may be accurately detected. On the other hand, the trailing edge is of a degraded wave form making its detection more difficult. Thus, it is difficult to accurately detect and measure the pulse width of such a heart pacemaker pulse.
In the above-noted prior circuits for detecting pulse widths, a differentiator circuit is used to detect the leading and trailing edges. Typically, there is no problem in detecting the occurrence of the leading edge of a pacer pulse, which under normal circumstances has a sufficiently fast rise time to actuate normal differentiator circuits to provide a defined output pulse. Similarly, prior art differentiator circuits are capable of providing an output corresponding to the trailing edge, even of a degraded pulse as shown in FIG. 4A. The problem then arises when the attenuated or degraded slope of the pulse occurring between the leading and trailing edges may have a sufficiently fast fall time such that the output of the differentiator circuit may have a sufficient amplitude so as to cause the associated threshold circuit to provide a premature output indicative of the trailing edge. As shown in FIG. 4A, the drooping waveform portion of the pacemaker pulse is essentially capacitive in nature being attenuated by the indicated expression. Therefore, the droop or attenuation associated with the detected artifact pulse is predictable, assuming that a reasonable range of capacitive coupling is made to the patient's heart. The worst contemplated case of attenuation is considered to be a decrease in impulse amplitude of 50% within a time period of 200 microseconds or 80% in 1 millisecond. In other words, the leading edge of a degraded pulse may have a time constant in the order of 3 or 4 microseconds, the trailing edge a time constant in the order of 70 microseconds, and the attenuated or drooping intermediate waveform portion may have a time constant in the order 620 microseconds. Though many multiples of either the leading or trailing edge, the time constant of the drooping portion may be sufficient to provide a premature indication of the trailing edge and therefore an incorrect indication of the pulse width.